Acetic acid is a recoverable solvent utilized in the production of terephthalic acid in many processes. See U.S. Pat. No. 4,769,487, British 1,583,755, Canadian 1,113,957, and Japanese 53-71034, 58-39812, 59-33579, 53-79836, and 56-4587.
Distillation has been widely used as a primary unit operation for acetic acid recovery in such processes, utilizing one or more towers to process a number of streams of varying concentrations of acetic acid with the purpose of recovering it for further use in the oxidation step. The products from the distillation tower are a bottom stream of concentrated acetic acid, and an overhead stream that ideally would be pure water.
Because of the high non-ideality of the system acetic acid/water and the equilibrium limitation in such a system, it is necessary to utilize a distillation tower with a high number of theoretical stages and high reflux ratio to be able to obtain reasonably low levels of acetic acid in the distilled water.
These involve a high investment cost because of the large dimensions of the required equipment and a high operating cost because of the high steam consumption involved. Furthermore, the traditional process scheme does not allow one to obtain economically a distillate completely free of acetic acid. This limitation, in turn, presents two main problems: a cost associated with the operation resulting from the acetic acid losses, and an environmental problem that is continually increasing, because of the ever more rigorous standards for acceptable levels of emission to the environment.
There has been an effort to look for alternatives to solve the problem. Resort has been had to azeotropic distillation, involving the addition of an additional component to the distillation tower to improve the relative volatility of the separation and reduce the separation requirements. This existing option provides some reduction in the operating costs, but it generates some additional operating and environmental problems.
A study of the vapor liquid equilibrium of the components acetic acid/water shows the difficulty in the dilute acetic region of reducing the acetic acid in the distilled water. The achievement of a reduction from the typical design value of 0.5 wt % acetic acid overhead to 0.1 wt % acetic acid overhead requires an increase in reflux ratio of about 10-15% or the inclusion of several additional theoretical stages to maintain the same reflux ratio.
Extraction of Acetic Acid in Dilute Streams
For several years it has been known that the use of liquid-liquid extraction is a way to recover acetic acid from dilute streams. Several extractive agents have been identified, and it is possible to economically recover acetic acid from streams containing 0.1 wt % acetic acid to 20% acetic acid. Some of the agents usually used are acetates, amines, ketones and phosphine oxides and mixtures thereof.
Once the extraction step is completed, a series of distillation steps are required to recover the acid and to recirculate the extractive agent back to the extraction stage. The level of impurities in the feed and the affinity of the particular extractive agent for water dictate the additional steps required in the system.